Finnsheep are known for their exceptionally high fecundity, and the breed has been imported to several countries to improve the fertility of many other breeds through cross-breeding. A recent doctoral dissertation produced volumes of new information about the function of genes related to the female fertility in domestic sheep and the genetic characteristics of native Finnsheep.
The doctoral dissertation of Kisun Pokharel MSc, research scientist at the Natural Resources Institute Finland (Luke), studied which genes contribute to the exceptional fertility of sheep, Finnsheep in particular, and how these genes function. It also studied how feeding affects the functioning of genes and therefore the female fecundity in sheep.
The doctoral dissertation explored two important reproductive events affecting fertility: the function of the ovaries before ovulation and the preimplantation period during which embryos remain unattached to the wall of the uterus (2–17 days after mating) by using the latest genome research methodology.
The study’s primary animal material consisted of Finnsheep and Texel ewes, and their first-generation F1 crosses (50% Finnsheep and 50% Texel).
Additional energy does not increase the ovulation rate in Finnsheep
The provision of additional energy has previously been found to increase the ovulation rate in ewes, i.e. the number of ova released during the oestrous cycle. The provision of additional energy through feeding had the most significant impact on the function of genes and the ovulation rate in Texel sheep, a mild impact on F1 crosses and no noticeable impact on Finnsheep.
Although the genome of F1 crosses comes equally from Finnsheep and Texel sheep, the function of genes in F1 crosses resembled that of Finnsheep more than being “intermediate” between the two breeds. This indicates that the genes originating from Finnsheep have been more active in the ova of F1 crosses than the genes originating from Texel sheep. This is called non-Mendelian epigenetic inheritance, as a result of which only genes originating from one parent are active.
In addition, a mutation at GDF9, a known reproductive gene assumed to increase the ovulation rate, was found in Finnsheep. Twice the number of corpora lutea were found in the ovaries of Finnsheep and F1 ewes than in Texel ewes, proof of the high ovulation rate in the native breed.
The genetic research indicated preliminarily that Finnsheep may be susceptible to the delayed implantation of embryos to the wall of the uterus, i.e. embryonic diapause. This tendency is useful for the reproduction of animals, especially in a low state of nutrition, whereby embryonic diapause postpones birth of offspring to a more suitable time. With regard to even-toed ungulates, embryonic diapause of 4–5 months has only been found in the European roe deer. TXNL1 (Thioredoxin-like protein 1) is one of the key genes in regulating embryonic diapause. Its genetic activity was noticeably high in Finnsheep and F1 ewes. This indirect gene-based finding may result from the high ovulation rate and long breeding season of Finnsheep.
Finnsheep have an effective immune system
The study produced evidence that Finnsheep have a genetically effective immune system, which improves defence against harmful bacteria and viruses. This may have an impact on exceptional fertility.
The doctoral dissertation also provided information about the regulation of genetic functions in sheep by examining short microRNA molecules that have an impact on the regulation of the function of genes, slowing their activity due to environmental impact, for example.
The study identified more than 500 different microRNA molecules in sheep. Previously, only 153 molecules had been identified. The comprehensive list of microRNA molecules and genes is a useful research resource in understanding the function of genes in the development of embryos, as well as the development of analysis methods for genome research.
Kisun Pokharel will defend his doctoral dissertation “Functional Fertility Genomics in Sheep” in the Faculty of Biological and Environmental Sciences at the University of Helsinki on 24 November 2020. Professor David MacHugh from University College Dublin will act as the opponent and professor Craig Primmer from the University of Helsinki as the custos.
Photos: Mervi Honkatukia